1. Field of the Invention
The present invention relates to a DC to DC converter and a related method thereof, and particularly relates to a DC to DC converter with load open detection and a related method thereof.
2. Description of the Prior Art
FIG. 1 is a circuit diagram illustrating a prior art LED driving circuit 100 for transforming an input voltage Vin to an output voltage Vout according to desired luminance to drive a plurality of LEDs 101. As shown in FIG. 1, the LED driving circuit 100 comprises a dimming control circuit 103, a hysteresis comparator 105, a comparator 106, a control circuit 107, an inverter 109, and a transforming circuit 120 comprising a NMOS 111, an inductance 113, a diode 115 and a capacitor 119. The dimming control circuit 103 is used for generating a first reference voltage Vref1 according to a second reference voltage Vref2 and a dimming control voltage DCV, which is used for adjusting the luminance of the LEDs 101. The hysteresis comparator 105 is used for comparing the first reference voltage Vref1 and a feedback voltage VFB proportional to the output voltage Vout to generate a comparing signal COS. As known by persons skilled in the art, the hysteresis comparator 105 is used for limiting the feedback voltage VFB between a region, instead of limiting the feedback voltage VFB to a specific value. Since the detail operation is well known, the detail description of the hysteresis comparator 105 is omitted for brevity. In this case, a loading current IL flows through the LEDs 101 and the resistor 117, therefore a voltage drop exists on the LEDs 101, and the feedback voltage VFB is generated accordingly.
The control circuit 107 is used for generating a control signal CS. The inverter 109 is used for inverting the control signal CS to control the NMOS 111. If the feedback voltage VFB is lower than the first reference voltage Vref1, the control circuit 107 makes the transforming circuit 120 generate a higher output voltage Vout. Similarly, if the feedback voltage VFB is higher than the first reference voltage Vref1, the control circuit 107 makes the transforming circuit 120 generate a lower output voltage Vout. Also, the hysteresis comparator 105, the comparator 106, the control circuit 107 and the transforming circuit 120 can be regarded as a DC to DC converter.
The comparator 106 is used for detecting a load open situation. If the circuit at the side of the LEDs 101 is open, the feedback voltage VFB is 0, so the comparator 106 will compare the feedback voltage VFB and a third reference voltage Vref3. If the feedback voltage VFB is lower than the third reference voltage Vref3, the comparator 106 will inform the control circuit 107 that a load open situation has occurred, and the control circuit 107 will turn off the NMOS 111. The third reference voltage Vref3 is normally 0.1V. Such a mechanism has some disadvantages, however.
The control circuit 107 can be a switching circuit or a logic circuit such as an AND gate. In this case, when the LED driving circuit 100 operates normally, the control signal CS is equal to comparing signal COS. If the feedback voltage VFB is lower than the third reference voltage Vref3, the comparator 106 will inform the control circuit 107 that a load open situation has occurred, the comparing signal COS can not pass through the control circuit 107, and the control signal CS will keep on logic high level, thus the NMOS 111 will be turned off In other words, the control circuit 107 will turn off the NMOS 111 by blocking the comparing signal COS.
FIG. 2 is a schematic diagram illustrating the relation of the first reference voltage Vref1, the second reference voltage Vref2, and the dimming control voltage DCV shown in FIG. 1. As shown in region A of FIG. 2, the first reference voltage Vref1 is proportional to the dimming control voltage DCV if the dimming control voltage DCV is smaller than a threshold voltage Vth (for example, 3.5V). In region B, the first reference voltage Vref1 is limited to the second reference voltage Vref2 if the dimming control voltage DCV is larger than a threshold voltage Vth. As described above, if the feedback voltage VFB is lower than the third reference voltage Vref3, it is determined that the load open situation has occurred. However, if the first reference voltage Vref1 determined by the dimming control voltage DCV in region A is smaller than the third reference voltage level Vref3, a normal situation may be determined to be a load open situation. Thus, this system is subject to errors.